Hi.
I vote for comments near the implementation.
I have been digging into the driver model code recently, but I have to admit it is unreadable because most of comments are placed in its header files. (and I am planning to send a patch to move comments to C sources.)
I really want to know "what does this function do?" and "How is it used?" things before I start to read the detailed implementation.
If they are written separately, I need to open two windows of my editor, one for reading the comments in a header file, the other for reading the implementation in a C source. I am really unhappy about that.
I guess people often use tag jump utilities. (I like GNU Global, someone may use ctags/etags, cscope, etc. I don't know..)
Such utilities allow us to jump over to the implementation place. If comments are not there, we have to look for comments by hand.
I think we should keep in our mind this: source files are much more read than written. I believe we put the readers' benefits at the top priority.